With the present world-wide emphasis on the energy crisis and the rapidly diminishing sources of oil, increased attention by both government and private organizations is being given to coal as a source of energy, especially for the generation of electricity. For the year 1972 the annual consumption of coal in the United States for the generation of electricity exceeded 348 million tons. This country has vast resources of coal for development as other sources of energy diminish.
Depending upon their origin, coals contain varying amounts of iron disulfide (iron disulfide is hereinafter referred to as pyrite whether crystallized as pyrite or marcasite) from which sulfur dioxide is formed as a combustion product when coal is burned. This is a tremendous disadvantage to the use of coal as an enerby source, particularly in view of the present emphasis on pollution control as illustrated by present federal emission control standards for sulfur dioxide. Illustrating the enormity of the sulfur dioxide emission problem is the fact that large transportation expenses are incurred by coal users in transporting Western and European coal of relatively low sulfur content long distances to supplant available high sulfur-containing coals in order to make compliance with sulfur dioxide emission standards possible when using coal as an energy source. At this time there are no effective means available which are commercially feasible for absorbing the large amounts of sulfur dioxide emitted by the combustion of coal to produce heat and electricity. Currently U.S. utilities in burning about 395 million tons of coal a year generate about 21 million tons of sulfur dioxide in the process. One solution to the problem is to separate the sulfur-bearing pyrite from the coal before it is burned.
Coals also contain, depending upon their origin, various amounts and kinds of minerals which form ash when the coal is burned. The ash also is a disadvantage to the use of coal as an energy source, since it contributes no energy value during combustion thereby diluting the calorific value of the coal, causes a waste disposal problem, and a potential air pollution problem.
The problem of separating pyrite or other impurities from raw coal is not new and a number of methods have been extensively tested over the years. Among these are methods which employ the difference in specific gravity between coal particles and the impurity particles or differences in their surface, electrostatic, chemical or magnetic properties. For one reason or another difficulties are encountered in making an efficient separation of pyrite or other impurities from coal which has been ground finely enough to substantially liberate impurity particles from coal particles. In water systems this difficulty is related to the slow settling rate of fine particles and in air systems to the large difference in specific gravity between air and the particles. However, for magnetic separations the magnetic attraction force acting on small magnetic particles is many times greater than the opposing separating force, which is usually a hydraulic drag and/or gravity force.
For the separation of pyrite or other impurities from raw coal the success of a magnetic process is dependent on some effective pre-treatment process for selectively enhancing the magnetic susceptibility of the pyrite or impurity particles. Coal particles alone are slightly diamagnetic while pyrite and many other mineral impurities are weakly paramagnetic; however, their paramagnetism is not sufficient to economically effect a separation from coal. However, effective beneficiation of coals can be made if the apparent magnetic susceptibility of pyrite or other impurities is increased. For pyrite it has been estimated that a sufficient increase in susceptibility can be achieved by converting less than 0.1 percent of pyrite in pyritic coal into ferromagnetic compounds of iron. ("Magnetic Separation of Pyrite from Coals," Bureau of Mines Report of Investigations 7181, P.1.)
In discussing the use of heat to enhance the paramagnetism of pyrite it is stated in the above report (P.1) that ferromagnetic compounds of iron are not formed in significant quantities at temperatures below 400.degree.C, and that such conversion occurs in sufficient quantities to effect beneficiation only at temperatures greater than 500.degree.C. As this is above the combustion point of coal, the use of heat to enhance magnetic susceptibility does not appear feasible. Further, other methods for enhancing the paramagnetism of pyrite to permit its separation from coal have not been encouraging.
Accordingly, it is a principal object of this invention to provide an economically feasible method for improving raw coal by enhancing the apparent magnetic susceptibility of pyrite or other impurities associated with but substantially liberated from the raw coal to the point where these impurities can be successfully separated from the coal by magnetic separators.
It has been found that pyrite reacts with iron carbonyls to form one or more compounds different from pyrite and having a magnetic susceptibility very much greater than the original pyrite. Although iron pentacarbonyl has proven effective in the reaction, it is obvious that other carbonyls, such as iron nonacarbonyl or a mixture of iron carbonyls would also be effective and the term "iron carbonyl" as used herein includes all carbonyls of iron and mixtures thereof. This discovery can be used to alter the surface of the pyrite by applying the carbonyl treatment so that the apparent magnetic susceptibility of the pyrite is increased. Pyrite particles that have been so treated can then be separated by magnetic processing from other materials which are inert to a surface treatment of iron carbonyl. Such a process has wide application in the field of mineral beneficiation.